1. Field of the Invention
This invention relates generally to impressed current anodes for the cathodic protection of metallic structures and relates particularly to an apparatus and method for causing substantially equal discharge of an impressed electronic current along the entire length of the anode.
2. Description of the Prior Art
In the past, it has been recognized that an underground metallic structure has been subjected to chemical or electrochemical attack which causes rust and other corrosion since the metallic structure normally includes both anodic and cathodic areas. A galvanic electric current normally flows from the ground to the cathodic areas so that substantially no corrosion occurs in these areas; however, an electric current flows from the anodic areas into the ground which promotes corrosion. It is known that a higher electrical current from an impressed current anode system embedded in a carbonaceous backfill environment and located in the area of the underground metallic structure causes the entire surface of the underground metallic structure to become cathodic and thereby substantially prevents corrosion.
Heretofore many efforts have been made to provide anodes and anode systems for the cathodic protection of metallic structures and these have included deep well anode systems, shallow well anode systems, and systems for use in water. Initially sacrificial anodes were provided which emitted a galvanic current and these sacrificial anodes slowly disintegrated so that the useful life of the anode was limited. Some efforts were made to extend the life of the sacrificial anodes by covering portions of the anode surface with a dielectric material. However, care was required to permit sufficient current to flow to prevent corrosion of the structure. Some examples of this type of prior art structure are shown in the U.S. Pat. Nos. to Douglas 2,855,358, Vixler 3,012,958 and Shutt 3,354,063.
In order to extend the effective life of a cathodic protection system and to insure that sufficient current was present at the metallic structure, anodes were provided which were electrically connected to a rectifier or the like so that an impressed electrical current which could be controlled to certain values was applied to the anodes. The anodes were made of iron, high silicon cast iron, steel, copper, graphite, magnetite, and other materials. Normally, in groundbeds, the anodes were embedded in a carbonaceous backfill material such as calcined petroleum coke, metallurgical coke, graphite and the like. An impressed current was applied to the anodes at a current density sufficient to cause the underground metallic structure to become cathodic. However, these anodes slowly deteriorated so that it was necessary to replace them every few years. An example of this type of structure is Tatum U.S. Pat. No. 3,725,669.
In a further effort to extend the life of the anodes, titanium and niobium anodes were provided which were partially or completely plated with a noble metal such as platinum or the like. In the partially plated type of structure, when an impressed current was applied to the anodes, the non-coated portions of the titanium or niobium did not discharge current because the substrate materials had a natural threshold voltage which caused the anode material to polarize and form a non-conducting film along the exposed exterior surfaces, while the current discharge occurred from the platenized surfaces into the carbonaceous backfill material or other electrolyte. This type of anode has been expensive but has had a longer life.
Some examples of this type of structure are the U.S. Pat. Nos. to Baum 1,477,499, Anderson 2,998,359, Krause 3,929,607, British Patent No. 866,577, and the following publications: Platinum Metals Review, Vol. 2, No. 2, April 1958, pages 45-47; Platinum Metals Review, Vol. 4, No. 1, January 1960, pages 15-17; Corrosion Technology, February 1960, page 50; Corrosion Technology, January 1962, pages 14-16; Corrosion Technology, February 1962, pages 38-40; Corrosion Prevention and Control, October 1962, pages 51, 52 & 54.
Generally, these prior art anodes and particularly the anodes used in groundbeds, have been long slender anodes having a length of from 9 inches (23 cm) to 8 feet (244 cm) and a diamter of 1 inch (2.54 cm) to 6 inches (15.24 cm) which included a length-to-diameter ratio in excess of one.
Many of these prior anodes have failed prematurely due to a phenomena known as end effect or penciling and the cause of this phenomena is not clear. The obvious problem caused by end effect is the consumption of the anode material, ordinarily at one or both ends, resulting in a shorter system life. A less obvious problem is the loss of the electrical connection to the anode while the majority of the anode remains intact. This is due to the fact that most of the anodes available have the electrical connection at one end of the anode. Loss of the connection to one anode in a system results in the inability to discharge any current from the affected anode. Assuming a constant current demand, this means that the remaining anodes of the system must contend with a higher current density which compounds the end effect phenomena resulting in a domino effect.
An early attempt to deal with end effect in deep well anodes involved stacking the anodes close together. This technique slowed the rate of attack on most of the anodes in the groundbed; however, end effect on the outer anodes tended to be magnified.
A later attempt involved the addition of extra anode material around the connection at the end of the anode. This technique only delayed the inevitable result.
A more recent attempt to negate the results of end effect involved locating the electrical connection in the center of the anode. This technique did not solve the problem of end effect but it extended the life of the anode since the connection area was the last area of the anode to be consumed due to end effect.